In the field of single crystal manufacturing technology, the materials for growing crystals include semiconductors, organic matter, inorganic matter (oxides), metals and superconductors, etc. The current methods for growing these crystals mainly include Czochralski method, Bridgman method and a gradient freeze method that is very similar to Bridgman method.
In general, the yield of Czochralski method is higher than that of gradient freeze method, but the number of defects caused by the thermal stress in the Czochralski method is larger. Therefore, it is more popular to use the Bridgman method and the gradient freeze method to grow compound crystals in addition to the silicon single crystals in recent years.
As to the Bridgman method and the gradient freeze method, the differences reside in that the Bridgman method moves a crucible in a furnace having a high temperature portion and a low temperature portion to change the melting temperature inside the crucible, and the gradient freeze method moves the crucible to change the melting temperature inside the crucible. Whether the Bridgman method or the gradient freeze method, a stable ambient temperature is provided for steadily growing single crystals regardless, and thus a more appropriate crystal growth condition is provided for manufacturing crystals with higher quality and less defects. However, the latent heat of solidification will be released in a crystal growth process. FIG. 1 shows the interface between the melting state and solid state of the single crystal growth according to the prior art. The interface 13 between the solid-state single crystal 11 and melting state 12 is usually depressed; wherein the melting state 12 could be for any of the foregoing single crystal growing materials, and the convective flow caused by gravitational force will create a doping 14 in the melting state 12 which is distributed and centralized at the center of the interface 13. In other words, an axial segregation and the radial segregation of the doping 14 are created and thus cause an excessively cold interface 13 or an interface breakdown. The direction of the gravitational force is indicated by Arrow G, and the direction of the convective flow is indicated by Arrow C. As described above, even though the heat transfer can be precisely controlled in the crystal growth process, it is still unable to eliminate the convective flow in the melting state. Therefore, the crystal so grown will be defective, and it is necessary to reduce the axial segregation and radial segregation in order to effectively control the distribution of the doping in the axial and radial directions. It is very important to effectively control the convective flow.
In the prior art, a magnetic field is added to reduce some of the accumulated doping in order to effectively reduce the impact of convective flow to the crystal growth. However, such arrangement not only makes the manufacturing difficult and the cost high, but also cannot effectively provide a magnetic field to the crystal growing area to control the crystal growth, and thus the method can only be applied for the solution being used as an electric conductor.
Please refer to FIG. 2 for the illustrative view of the prior-art centrifugal crystal growth system. In recent years, a centrifugal method has been developed to reduce some of the convective flow and further improve the axial segregation. This method uses a large centrifuge 21 to produce a centrifugal force on a crucible 22. In most cases, a free-swing rotation is adopted for such operation. In other words, the direction of the composite acceleration of the centrifugal force and the gravitational fore is parallel to the axis of the crucible. However, the aforementioned operation method has not fully used the centrifugal force and the Coriolis force, and will cause a three-dimensional flow of the melting state 23 and increase the axial segregation of the doping 24. A coaxial rotation can be used to achieve the same effect. Further, the accelerated crucible rotation technique (ACRT) is used by changing rotation speed, and an Eckman flow near the interface and a Taylor flow along the ampoule wall are added and mixed to produce a long-cycle unstable growth and impurity fluctuated distribution. To effectively form the Eckman (convective) layer, the cycle for the change of rotation is long and usually falls in the range from several seconds to several minutes.
In the R.O.C. Patent Publication No. 500839 entitled “rational directional solidification crystal growth system and method”, a furnace, a crucible and a rational mounting device are disclosed. The rational mounting device holds and rotates the crucible, and the tangential speed of the rotation of the crucible is not less than 5π/3 cm/sec. Therefore, it is known that the rational mounting device rotates the crucible in hope of providing a certain centrifugal force perpendicular to the gravitational force to the raw materials and the doping therein and further to eliminate the recession disposed at the center of the interface due to the accumulated doping and enhance the interface stability. That invention also produces a convective flow in the opposite direction of the natural convective flow to eliminate the natural convective flow in order to improve the doping distribution (both in the axial and radial directions) and the single crystal quality. However, a simple rotation in one direction really cannot meet the desired efficiency. Therefore, an effective way of reducing the convective flow phenomenon (which also reduces the axial segregation) and eliminating the central recession and breakdown at the center of the interface as to reduce the axial segregation and the radial segregation and further to avoid the excessively cold interface and the interface breakdown is a subject that deserves immediate attention.